Ligation of T cell receptor/CD3 complexes induces programmed cell death, or apoptosis, in immature thymocytes and many T cell hybridomas. While it has been demonstrated that T cell receptor‐mediated apoptosis requires an increase in intracellular calcium concentration, the specific calcium‐dependent signalling events leading to cell death are poorly defined. We have previously shown that T cell receptor/CD3‐mediated induction of apoptosis in a murine T cell hybridoma is inhibited by the immunosuppressive drugs cyclosporin A (CsA) and FK506. Recently, it has been determined that these agents inhibit the activity of calcineurin, a calcium‐ and calmodulin‐dependent serine/threonine phosphatase. Using an assay which measures calcineurin activity in cell lysates, we find that calcineurin‐dependent dephosphorylation of a phosphopeptide substrate is potently inhibited in hybridomas treated with CsA or FK506. Drug dose‐response analyses indicate that the level of cellular calcineurin activity correlates closely with the ability of these cells to undergo apoptosis. Thus, calcineurin appears to be a critical mediator of T cell receptor/CD3 signalling leading to programmed cell death in T cell hybridomas.